Polymeric piezoelectric film and manufacturing method thereof

ABSTRACT

A polymeric piezoelectric film, including a helical chiral polymer (A) having a weight average molecular weight of from 50,000 to 1,000,000 and optical activity, in which, in the film: a crystallinity given by a DSC method is from 20% to 80%; a standardized molecular orientation MORc is from 3.5 to 15.0 when a reference thickness measured by a microwave transmission-type molecular orientation meter is 50 μm; and when a direction parallel to a phase difference streak is a direction X, a direction perpendicular to the direction X and parallel to a main plane of a film is a direction Y, and the phase difference streak is evaluated by an evaluation method A, per a length of 1,000 mm in the direction Y, a number of phase difference streaks with an evaluation value of 60 or more is 0, and a sum of the evaluation values of phase difference streaks with an evaluation value of 20 or more is 1000 or less.

TECHNICAL FIELD

The present invention relates to a polymeric piezoelectric film and amanufacturing method thereof.

BACKGROUND ART

Conventionally, PZT (PbZrO₃—PbTiO₃-based solid solution) which is aceramic material has been widely used as a piezoelectric material. SincePZT contains lead, however, as a piezoelectric material, polymericpiezoelectric materials (polymeric piezoelectric films) having lowenvironmental load and being rich in flexibility are now being used.

Currently known polymeric piezoelectric materials are poled polymersrepresented by Nylon 11, polyvinyl fluoride, polyvinyl chloride,polyurea, polyvinylidene fluoride (β-type) (PVDF), vinylidenefluoride-trifluoroethylene copolymer (P(VDF-TrFE)) (75/25), and thelike.

In recent years, attention has been given to using polymers with opticalactivity, such as polylactic acids, in addition to the above-describedpolymeric piezoelectric materials. Polylactic acid-type polymers areknown to express piezoelectricity only by a mechanically, stretchingoperation.

The piezoelectricity of polymer crystals such as polylactic acids amongthe polymers with the optical activity are caused by permanent dipoleswith C═O bonds that are present in a helical axis direction. Inparticular, the polylactic acid has the low volume fraction of sidechains based on that of main chains and the high percentage of permanentdipoles per volume, so that it may be said that it is an ideal polymeramong the polymers having helical chirality. The polylactic acid thatexpresses the piezoelectricity only by the stretching treatment does notrequire poling treatment and is known to have a piezoelectric modulusthat does not decrease for several years.

As described above, since polylactic acid has various piezoelectriccharacteristics, polymeric piezoelectric materials using variouspolylactic acids have been reported (see, for example, Patent Documents1

-   Patent Document 1 Japanese Patent Application Laid-Open (JP-A. No.    H05-152638-   Patent Document 2 JP-A No. 2005-213376-   Patent Document 3 JP-A No. 2014-086703

SUMMARY OF INVENTION Technical Problem

Incidentally, in order to develop piezoelectricity, the polymericpiezoelectric film needs to orient the molecular chain in one direction.For example, Patent Document 3 describes a uniaxially stretched film inwhich molecular chains are oriented in a stretching direction bystretching in the longitudinal direction. In such a uniaxially stretchedfilm, a streak (phase difference streak) tends to occur in a directionparallel to a stretching direction (a direction in which molecularchains are oriented).

Further, in the case of using uniaxially stretched films as described inPatent Document 3, they are easily torn parallel to the stretchingdirection, and they have a drawback in that the tear strength in acertain direction is low. Hereinafter, the tear strength in a certaindirection is also referred to as “longitudinal tear strength”.

Here, in order to obtain a polymeric piezoelectric film having easyalleviation of a streak and high longitudinal tear strength, generally,longitudinal and lateral magnifications are increased, and thelongitudinal and lateral magnifications are brought close to the sameextent to perform stretching. However, when the longitudinal and lateralmagnifications are brought close to the same extent, the orientation ofmolecular chains deteriorates and the piezoelectricity of a polymericpiezoelectric film deteriorates.

On the other hand, the present inventors intensively studied to findthat a piezoelectric polymer film having excellent longitudinal tearstrength while maintaining the piezoelectricity can be obtained byreducing phase difference streaks, thereby completing the presentinvention.

An object of the present invention is to provide a polymericpiezoelectric film having reduced phase difference streaks and excellentlongitudinal tear strength while maintaining piezoelectricity and amethod of manufacturing the same.

Solution to Problem

Specific means for achieving the problems are as follows.

<1> A polymeric piezoelectric film, comprising a helical chiral polymer(A) having a weight average molecular weight of from 50,000 to 1,000,000and optical activity, wherein, in the film: a crystallinity given by aDSC method is from 20% to 80%; a standardized molecular orientation MORcis from 3.5 to 15.0 when a reference thickness measured by a microwavetransmission-type molecular orientation meter is 50 μm; and when adirection parallel to a phase difference streak is a direction X, adirection perpendicular to the direction X and parallel to a main planeof a film is a direction Y, and the phase difference streak is evaluatedby an evaluation method A, per a length of 1,000 mm in the direction Y,a number of phase difference streaks with an evaluation value of 60 ormore is 0, and a sum of evaluation values of phase difference streakswith an evaluation value of 20 or more is 1000 or less, the evaluationmethod A comprising:

(a) with respect to the direction Y, acquiring in-plane phase differencedata of a film at intervals of 0.143 mm to obtain an in-plane phasedifference profile;(b) performing fast Fourier transformation on the obtained in-planephase difference profile, removing low frequency components using0.273/mm as a cutoff frequency, and then performing inverse Fouriertransformation;(c) calculating slopes of two adjacent points with respect to thein-plane phase difference profile after inverse Fourier transformationand converting the slopes into a slope profile; and(d) taking a height from a bottom point of a valley of the obtainedslope profile to an apex of a mountain adjacent to the valley as anevaluation value of a phase difference streak.

<2> The polymeric piezoelectric film according to <1>, wherein, whenevaluated by the evaluation method A, per a length of 1,000 mm in thedirection Y, a number of phase difference streaks with an evaluationvalue of 40 or more is 0, and a sum total of evaluation values of phasedifference streaks with an evaluation value of 20 or more is 200 orless.

<3> The polymeric piezoelectric film according to <1> or <2>, wherein,when evaluated by the evaluation method A, per a length of 1,000 mm inthe direction Y, a number of phase difference streaks with an evaluationvalue of 20 or more is 0, and a sum total of evaluation values of phasedifference streaks with an evaluation value of 20 or more is 0.

<4> The polymeric piezoelectric film according to any one of <1> to <3>,wherein internal haze for visible light is 50% or less, and apiezoelectric constant d₁₄ measured by a stress-charge method at 25° C.is 1 pC/N or more.

<5> The polymeric piezoelectric film according to any one of <1> to <4>,wherein internal haze for visible light is 13% or less.

<6> The polymeric piezoelectric film according to any one of <1> to <5>,wherein the helical chiral polymer (A) is a polylactic acid-type polymerhaving a main chain containing a repeating unit represented by thefollowing Formula (1).

<7> The polymeric piezoelectric film according to any one of <1> to <6>,wherein a content of the helical chiral polymer (A) is 80% by mass ormore.

<8> The polymeric piezoelectric film according to any one of <1> to <7>,wherein a product of the standardized molecular orientation MORc and thecrystallinity is from 75 to 700.

<9> The polymeric piezoelectric film according to any one of <1> to <8>,wherein internal haze for visible light is 1.0% or less.

<10> The polymeric piezoelectric film according to any one of <1> to<9>, the film containing from 0.01 parts by mass to 10 parts by mass ofa stabilizer having one or more functional groups selected from thegroup consisting of a carbodiimide group, an epoxy group, and anisocyanate group, the stabilizer having a weight average molecularweight of 200 to 60,000 (B) based on 100 parts by mass of the helicalchiral polymer (A).

<11> A method of manufacturing the polymeric piezoelectric filmaccording to any one of <1> to <10> the method comprising: a step ofextruding a composition containing the helical chiral polymer (A) from aT-die having a lip tip edge radius of from 0.001 mm to 0.100 trim at anextrusion temperature of from 200° C. to 230° C. to form the compositioninto a film; and a step of stretching the formed film.

Advantageous Effects of Invention

According to the present invention, a polymeric piezoelectric filmhaving reduced phase difference streaks and excellent longitudinal tearstrength while maintaining piezoelectricity; and a method ofmanufacturing the same can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating an in-plane phase difference profile of afilm acquired for a polymeric piezoelectric film of Comparative Example1.

FIG. 2 is a graph illustrating an in-plane phase difference profile of afilm after inverse Fourier transformation (after removal of lowfrequency components) of the polymeric piezoelectric film of ComparativeExample 1.

FIG. 3 is a graph illustrating a slope profile of the polymericpiezoelectric film of Comparative Example 1.

FIG. 4 is a graph illustrating evaluation values of phase differencestreaks for a polymeric piezoelectric film of Example 2.

FIG. 5 is a graph illustrating evaluation values of phase differencestreaks for a polymeric piezoelectric film of Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

Hereinafter, one embodiment of the polymeric piezoelectric film of thepresent invention will be described.

A numeral value range represented by “(a value) to (a value)” means arange including the numeral values represented before and after “(avalue) to (a value)” as a lower limit value and an upper limit value,respectively.

The term “film” is herein a concept that includes not only what isgenerally called “film” but also what is commonly called “sheet”.

Herein, a film surface means a principal plane of a film. Here, the t“principal plane” refers to a plane having the largest area among thesurfaces of the polymeric piezoelectric film. The polymericpiezoelectric film of the present embodiment may have two or moreprincipal planes. For example, when the polymeric piezoelectric film hastwo plates A with a size of 10 mm×0.3 mm, two plates 13 with a size of 3mm×0.3 mm, and two plates C with a size of 10 mm×3 mm, the principalplane of the polymeric piezoelectric film is the plate C, and the filmhas two principal planes.

Herein, the term “MD direction” means a flow direction of a film(Machine Direction); and the term “TD direction” means a directionperpendicular to the MD direction and parallel to the principal plane ofthe film (Transverse Direction).

<Polymeric Piezoelectric Film>

A polymeric piezoelectric film according to one embodiment of theinvention is a polymeric piezoelectric film comprising a helical chiralpolymer (A) having a weight average molecular weight of from 50,000 to1,000,000 and optical activity, wherein, in the film: a crystallinitygiven by a DSC method is from 20% to 80%; a standardized molecularorientation MORc is from 3.5 to 15.0 when a reference thickness measuredby a microwave transmission-type molecular orientation meter is 50 μm;and when a direction parallel to a phase difference streak is adirection X, a direction perpendicular to the direction X and parallelto a main plane of a film is a direction Y, and the phase differencestreak is evaluated by an evaluation method A; per a length of 1,000 mmin the direction Y, the number of phase difference streaks with anevaluation value of 60 or more is 0, and the sum of evaluation values ofphase difference streaks with an evaluation value of 20 or more is 1000or less, the evaluation method A comprising:

(a) with respect to the direction Y, acquiring in-plane phase differencedata of a film at intervals of 0.143 mm to obtain an in-plane phasedifference profile;(b) performing fast Fourier transformation on the obtained in-planephase difference profile, removing low frequency components using0.273/mm as a cutoff frequency, and then performing inverse Fouriertransformation;(c) calculating slopes of two adjacent points with respect to thein-plane phase difference profile after inverse Fourier transformationand converting the slopes into a slope profile; and(d) taking height from a bottom point of a valley of the obtained slopeprofile to an apex of a mountain adjacent to the valley as an evaluationvalue of a phase difference streak.

When a polymeric piezoelectric film has the above-describedconfiguration, phase difference streaks are reduced and the longitudinaltear strength is excellent while maintaining the piezoelectricity.

More specifically, when the phase difference streak is evaluated by theevaluation method A, since, per a length of 1,000 mm in the direction Y,the number of phase difference streaks with an evaluation value of 60 ormore is 0 and the total sum of the evaluation values of the phasedifference steaks with an evaluation value of 20 or more is 1,000 orless, the phase difference streaks of the polymeric piezoelectric filmare reduced, and as a result, a polymeric piezoelectric film havingexcellent longitudinal tear strength while maintaining piezoelectricitycan be provided.

That the tear strength in a certain direction deteriorates isoccasionally expressed herein as “longitudinal tear strengthdeteriorates”, and a situation where the tear strength in a certaindirection is low, is occasionally expressed herein as “longitudinal tearstrength is low”.

That a phenomenon of deterioration of the tear strength in a certaindirection is suppressed, is occasionally expressed herein as“longitudinal tear strength is improved”, and a situation where thephenomenon of deterioration of the tear strength in a certain directionis suppressed, is occasionally expressed as “longitudinal tear strengthis high” or “excellent longitudinal tear strength”.

The polymeric piezoelectric film contains a helical chiral polymer (A)having a weight average molecular weight (Mw) of 50,000 to 1,000,000.When the helical chiral polymer (A) has a weight average molecularweight of 50,000 or more, the mechanical strength of the helical chiralpolymer (A) as a molded body improves. When the weight average molecularweight of the helical chiral polymer (A) is 1,000,000 or less, theformability when forming the polymeric piezoelectric film by molding(for example, extrusion molding) is improved.

In the polymeric piezoelectric film, the crystallinity obtained by theDSC method is from 20% to 80%. Therefore, the polymeric piezoelectricfilm has a favorable balance between the piezoelectricity, transparencyand longitudinal tear strength, and since whitening or breaking is lesslikely to occur when stretching the polymeric piezoelectric film, it iseasy to manufacture the film.

More specifically, when the crystallinity is 20% or more, thepiezoelectricity of the polymeric piezoelectric film is maintained high,and when the crystallinity is 80% or less, deterioration of thelongitudinal tear strength and transparency of the polymericpiezoelectric film can be suppressed.

The polymeric piezoelectric film has a standardized molecularorientation MORc of from 3.5 to 15.0.

When the standardized molecular orientation MORc is 3.5 or more, thereare many molecular chains (for example, polylactic acid molecularchains) of the helical chiral polymer (A) having optical activityarranged in the stretching direction, and as a result, the rate at whichoriented crystals are generated increases, and the polymericpiezoelectric film can exhibit a high piezoelectricity.

When the standardized molecular orientation MORc is 15.0 or less, thelongitudinal tear strength of the polymeric piezoelectric film isimproved.

[Evaluation Method A]

In a polymeric piezoelectric film according to the present embodiment, aphase difference streak is evaluated by an evaluation method A, per alength of 1,000 mm in the direction Y, the number of phase differencestreaks with an evaluation value of 60 or more is 0, and the sum of theevaluation values of phase difference streaks with an evaluation valueof 20 or more is 1000 or less. Therefore, the polymeric piezoelectricfilm has reduced streaks, and as a result, it has excellent longitudinaltear strength while maintaining piezoelectricity.

Hereinafter, the evaluation method A which is a method of evaluating thephase difference streaks of the polymeric piezoelectric film accordingto the present embodiment will be described. The evaluation method A isperformed by the following procedures (a) to (d).

(a) With respect to the direction Y, in-plane phase difference data of afilm is acquired at intervals of 0.143 mm to obtain an in-plane phasedifference profile.(b) Fast Fourier transformation is performed on the obtained in-planephase difference profile, low frequency components are removed using0.273/mm as a cutoff frequency, and then inverse Fourier transformationis performed.(c) Slopes of two adjacent points with respect to the in-plane phasedifference profile after inverse Fourier transformation are calculatedand the slopes are converted into a slope profile.(d) A height from a bottom point of a valley of the obtained slopeprofile to an apex of a mountain adjacent to the valley is taken as anevaluation value of a phase difference streak.

First, in the above-described (a), in-plane phase difference data (phasedifference amount) of a film is acquired for a direction (a direction Y,for example, a TD direction) perpendicular to a direction (a directionX, for example, an MD direction) parallel to a phase difference streak(for example, fine streak-like irregularities of nm order occurring inthe MD direction which is a direction of a flow of the film), andparallel to a main plane of the film at intervals of 0.143 mm to obtainan in-plane phase difference profile. The in-plane phase difference dataof the film can be obtained, for example, by using a wide rangebirefringence evaluation system “WPA-100” manufactured by PhotonicLattice, Inc. The in-plane phase difference data (phase differenceamount) of the film is the product of the birefringence and thethickness, and assuming that the birefringence is constant, the amountof the phase difference is proportional to the thickness.

In the above-described (b), the in-plane phase difference profileobtained by the above-described (a) is subjected to fast Fouriertransformation, low frequency components are removed using 0.273/mm as acutoff frequency, and then inverse Fourier transformation is performed.Here, a high frequency component of the in-plane phase differenceprofile is caused by the phase difference streaks of the film, and a lowfrequency component of the in-plane phase difference profile is causedby the thickness unevenness (undulation) of the film, Therefore, byremoving low frequency components of the in-plane phase differenceprofile, only high frequency components caused by the phase differencestreaks of the film can be extracted.

Next, in the above-described (c), the slope of two adjacent points inthe in-plane phase difference profile after the inverse Fouriertransformation is calculated and converted into a slope profile. Then,in the above-described (d), the height from the bottom point of a valleyof the obtained slope profile to the apex of a mountain adjacent to thevalley is obtained, and the height is taken as an evaluation value of aphase difference streak. This evaluation value of a phase differencestreak corresponds to the intensity of a phase difference streak, andthe higher the numerical value is, the more conspicuous phase differencestreak is generated, and therefore, the numerical value is preferablylow. Further, the sum of evaluation values of phase difference streaksper a length of 1,000 mm in the direction Y corresponds to the influenceof a phase difference streak on the surface of a film, and the higherthe numerical value is, the more the phase difference streaks aregenerated in a wider range, or the more conspicuous phase differencestreaks are generated, and therefore, the numerical value is preferablylow.

In the polymeric piezoelectric film according to the present embodiment,when evaluating the phase difference streak by the evaluation method A,per a length of 1,000 mm in the direction Y, it is preferable that thenumber of phase difference streaks with an evaluation value of 40 ormore is 0 and the sum of evaluation values of phase difference streakswith an evaluation value of 20 or more is 200 or less, and it is morepreferable that the number of phase difference streaks with anevaluation value of 20 or more is 0 and the sum of evaluation values ofphase difference streaks with an evaluation value of 20 or more is 0. Bythis, the phase difference streaks of the polymeric piezoelectric filmare further reduced, and as a result, the film is more excellent in thelongitudinal tear strength while more favorably maintaining thepiezoelectricity.

In the above-described evaluation method A, a phase difference streakand the sum of phase difference streaks based on the length of 1,000 mmin the direction Y are evaluated, and, in a polymeric piezoelectric filmwhose length in the direction Y is less than 1,000 mm or whose length inthe direction Y exceeds 1,000 mm, the number of phase difference streakswith an evaluation value of 60 or more and the sum of evaluation valuesof phase difference streaks with an evaluation value of 20 or more areconverted into values per a length of 1,000 mm in the direction Y,respectively. For example, in a polymeric piezoelectric film having alength of 500 mm in the direction Y, the obtained number of phasedifference streaks with an evaluation value of 60 or more and total sumof the evaluation values of phase difference streaks with an evaluationvalue of 20 or more are respectively doubled to be converted into valuesper 1,000 mm length in the direction Y.

[Optically Active Helical Chiral Polymer (A)]

An optically active helical chiral polymer (A) (hereinafter, alsoreferred to as “helical chiral polymer (A)”) refers to a polymer with aweight average molecular weight of from 50,000 to 1,000,000 having ahelical molecular structure and having molecular optical activity.

Examples of the helical chiral polymer (A) include polypeptide,cellulose, a cellulose derivative, a polylactic acid-type polymer,polypropylene oxide, and poly(β-hydroxybutyric acid). Examples of thepolypeptide include poly(γ-benzyl glutarate), and poly(γ-methylglutarate). Examples of the cellulose derivative include celluloseacetate, and cyanoethyl cellulose.

The optical purity of the helical chiral polymer (A) is preferably95.00% ee or higher, more preferably 97.00% ee or higher, furtherpreferably 99.00% ee or higher, and particularly preferably 99.99% ee orhigher from a viewpoint of enhancing the piezoelectricity of a polymericpiezoelectric film. Desirably, the optical purity of the opticallyactive polymer is 100.00% ee. It is presumed that, by selecting theoptical purity of the helical chiral polymer (A) in the above range,packing of a polymer crystal exhibiting piezoelectricity becomes denserand as the result the piezoelectricity is improved.

The optical purity of the helical chiral polymer (A) in the presentembodiment is a value calculated according to the following formula:

Optical purity (% ee)=100×|L-form amount−D-form amount|/(L-formamount+D-form amount);

Specifically, the optical purity is a value obtained by multiplying(multiplying) ‘the value obtained by dividing (dividing) “the amountdifference (absolute value) between the amount [% by mass] of helicalchiral polymer (A) in L-form and the amount [% by mass] of helicalchiral polymer (A) in D-form” by “the total amount of the amount [% bymass] of helical chiral polymer (A) in L-form and the amount [% by mass]helical chiral polymer (A) in D-form” by ‘100’.

For the L-form amount [% by mass] of the helical chiral polymer (A) andthe D-form amount [% by mass] of the helical chiral polymer (A), valuesto be obtained by a method using high performance liquid chromatography(HPLC) are used. Specific particulars with respect to a measurement willbe described below.

Among the above helical chiral polymers (A), a polymer with the mainchain containing a repeating unit according to the following Formula (1)is preferable from a viewpoint of enhancement of the optical purity andimproving the piezoelectricity.

Examples of a compound with the main chain containing a repeating unitrepresented by the Formula (1) include a polylactic acid-type polymer.Among others, polylactic acid is preferable, and a homopolymer ofL-lactic acid (PLLA) or a homopolymer of D-lactic acid (PDLA) is mostpreferable.

The polylactic acid-type polymer means “polylactic acid”, a “copolymerof one of L-lactic acid or D-lactic acid, and a copolymerizablemulti-functional compound”, or a mixture of the two. The “polylacticacid” is a polymer linking lactic acid by polymerization through esterbonds into a long chain, and it is known that polylactic acid can beproduced by a lactide process via a lactide, a direct polymerizationprocess, by which lactic acid is heated in a solvent under a reducedpressure for polymerizing while removing water; or the like. Examples ofthe “polylactic acid” include a homopolymer of L-lactic acid, ahomopolymer of D-lactic acid; a block copolymer including a polymer ofat least one of L-lactic acid and D-lactic acid, and a graft copolymerincluding a polymer of at least one of L-lactic acid and D-lactic acid.

Examples of the “copolymerizable multi-functional compound” include ahydroxycarboxylic acid, such as glycolic acid, dimethylglycolic acid,3-hydroxybutyric acid, 4-hydroxybutyric acid, 2-hydroxypropanoic acid,3-hydroxypropannoic acid, hydroxyvaleric acid, 3-hydroxyvaleric acid,4-hydroxyvaleric acid, 5-hydroxyvaleric acid, 2-hydroxycaproic acid,3-hydroxycaproic acid; 4-hydroxycaproic acid; 5-hydroxycaproic acid,6-hydroxycaproic acid, 6-hydroxymethylcaproic acid, and mandelic acid; acyclic ester, such as glycolide, β-methyl-δ-valerolactone,γ-valerolactone, and ε-caprolactone; a polycarboxylic acid, such asoxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,pimelic acid, azelaic acid, sebacic acid, undecanedioic acid,dodecanedioic acid; and terephthalic acid, and an anhydride thereof; apolyhydric alcohol, such as ethylene glycol, diethylene triethyleneglycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol,1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,9-nonanediol, 3-methyl-1,5-pentanediol, neopentylglycol,tetramethylene glycol, and 1,4-hexanedimethanol; a polysaccharide suchas cellulose; and an aminocarboxylic acid such as α-amino acid.

Examples of the above “copolymerizable polyfunctional compound” includea compound described in paragraph 0028 of WO 2013/054918.

Examples of the “copolymer of one of L-lactic acid or D-lactic acid, anda copolymerizable polyfunctional compound” include a block copolymer ora graft copolymer having a polylactic acid sequence, which can form ahelical crystal.

The concentration of a structure derived from a copolymer component inthe helical chiral polymer (A) is preferably 20 mol % or less. Forexample, when the helical chiral polymer (A) is a polylactic acid-typepolymer, with respect to the total number of moles of a structurederived from lactic acid and a structure derived from a compoundcopolymerizable with lactic acid (copolymer component) in the polylacticacid-type polymer, the copolymer component is preferably 20 mol % orless.

The helical chiral polymer (A) (for example, polylactic acid-typepolymer) can be produced, for example, by a method of obtaining thepolymer by direct dehydration condensation of lactic acid, as describedin JP-A No, S59-096123 and JP-A No. H07-033861, or a method of obtainingthe same by a ring-opening polymerization of lactide, which is a cyclicdimer of lactic acid, as described in U.S. Pat. No. 2,668,182 and U.S.Pat. No. 4,057,357.

In order to make the optical purity of the helical chiral polymer (A)(for example, polylactic acid-type polymer) obtained by any of theproduction processes to 95.00% ee or higher, for example, when apolylactic acid is produced by a lactide process, it is preferable topolymerize lactide, whose optical purity has been enhanced to 95.00% eeor higher by a crystallization operation.

(Weight Average Molecular Weight of Helical Chiral Polymer (A))

The weight average molecular weight (Mw) of the helical chiral polymer(A) used in the present embodiment is from 50,000 to 1,000,000.

When the weight average molecular weight of the helical chiral polymer(A) is 50,000 or higher, the mechanical strength of a molded body fromthe helical chiral polymer (A) improves. The weight average molecularweight of the helical chiral polymer (A) is preferably 100,000 orhigher, and more preferably 150,000 or higher, from a viewpoint offurther improving the mechanical strength when formed into a moldedbody.

When the weight average molecular weight of the helical chiral polymer(A) is 1,000,000 or less, moldability when a polymeric piezoelectricfilm is obtained by molding (for example, extrusion molding) improves.The weight average molecular weight of the helical chiral polymer (A) ispreferably 800,000 or less, and more preferably 300,000 or less, from aviewpoint of further improving the formability at the time of obtaininga polymeric piezoelectric film.

The molecular weight distribution (Mw/Mn) of the helical chiral polymer(A) is preferably from 1.1 to 5, more preferably from 1.2 to 4, andfurther preferably from 1.4 to 3, from a viewpoint of the strength of apolymeric piezoelectric film. The weight average molecular weight Mw andthe molecular weight distribution (Mw/Mn) of a polylactic acid polymerare measured using a gel permeation chromatograph (GPC) by the followingGPC measuring method.

—GPC Measuring Apparatus—

GPC-100 manufactured by Waters Corp.

—Column—

Shodex LF-804 manufactured by Showa Denko K. K,

—Preparation of Sample—

A helical chiral polymer (A) is dissolved in a solvent (for example,chloroform) at 40° C. to prepare a sample solution with theconcentration of 1 mg/mL.

—Measurement Condition—

A sample solution 0.1 mL is introduced into the column at a temperatureof 40° C. and a flow rate of 1 mL/min by using a solvent [chloroform].

The sample concentration in a sample solution separated by the column ismeasured by a differential refractometer. Based on polystyrene standardsamples, a universal calibration curve is created and the weight averagemolecular weight (Mw) and the molecular weight distribution (Mw/Mn) of ahelical chiral polymer (A) are calculated.

For a polylactic acid-type polymer which is an example of a helicalchiral polymer (A), a commercial polylactic acid may be used, andexamples thereof include PURASORB (PD, PL) manufactured by PuracCorporate, LACER (H-100, H-400) manufactured by Mitsui Chemicals, Inc.,and Ingeo™ biopolymer manufactured by NatureWorks LLC.

When a polylactic acid-type polymer is used as the helical chiralpolymer (A) and in order to make the weight average molecular weight(Mw) of the polylactic acid polymer 50,000 or higher, it is preferableto produce the helical chiral polymer (A) by a lactide process, or adirect polymerization process.

A polymeric piezoelectric film of the present embodiment may containonly one kind of the helical chiral polymer (A), or may contain two ormore kinds thereof.

Although there is no particular restriction on a content (if two or morekinds are used, the total content; hereinafter holds the same) of thehelical chiral polymer (A) in a polymeric piezoelectric film of thepresent embodiment, 80% by mass or more with respect to the total massof the polymeric piezoelectric film is preferable.

When the content is 80% by mass or more, a piezoelectric constant tendsto improve.

[Stabilizer (B)]

A polymeric piezoelectric film of the present embodiment may contain, asa stabilizer (B), a compound which has at least one functional groupselected from the group consisting of a carbodiimide group, an epoxygroup, and an isocyanate group and whose weight average molecular weightis from 200 to 60,000. By this, the moisture and heat resistance of apolymeric piezoelectric film is further improved.

Further, as the stabilizer (B), the polymeric piezoelectric filmpreferably has at least one functional group selected from the groupconsisting of a carbodiimide group, an epoxy group and an isocyanategroup in one molecule.

For the stabilizer (B), “stabilizer (B)” described in paragraphs 0039 to0055 of WO 2013/054918 A can be used.

Examples of a compound having, in one molecule, a carbodiimide group(carbodiimide compound) which can be used as the stabilizer (B) includea monocarbodiimide compound, a polycarbodiimide compound, and a cycliccarbodiimide compound.

For the monocarbodiimide compound, dicyclohexylcarbodiimide,diisopropylphenylcarbodiimide, or the like is suitable.

As the polycarbodiimide compound, polycarbodiimide compoundsmanufactured by various methods can be used. Polycarbodiimide compoundsmanufactured by conventional methods of manufacturing a polycarbodiimide(for example, U.S. Pat. No. 2,941,956, Japanese Patent Publication(JP-B) No. S47-33279, J. Org. Chem. 28, 2069-2075 (1963), ChemicalReview 1981, Vol. 81, No. 4, p619-621) can be used. Specifically, acarbodiimide compound described in Japanese Patent No. 4084953 can bealso used.

Examples of the polycarbodiimide compound includepoly(4,4′-dicyclohexylmethanecarbodiimide),poly(N,N′-di-2,6-diisopropylphenylcarbodiimide), andpoly(1,3,5-triisopropylphenylene-2,4-carbodiimide.

The cyclic carbodiimide compound can be synthesized based on a methoddescribed in JP-A No. 2011-256337, or the like.

As the carbodiimide compound, a commercially available product may beused. Examples thereof include B2756 (trade name) manufactured by TokyoChemical Industry Co., CARBODILITE LA-1 (trade name) manufactured byNisshinbo Chemical Inc., and Stabaxol P, Stabaxol P400, and Stabaxol I(all are trade names) manufactured by Rhein Chemie GmbH.

Examples of a compound having, in one molecule, an isocyanate group(isocyanate compound) which can be used as the stabilizer (B) include3-(triethoxysilyl)propyl isocyanate, 2,4-tolylene diisocyanate,2,6-tolylene diisocyanate, m-phenylene diisocyanate, p-phenylenediisocyanate, 4,4′-diphenylmethane diisocyanate, 2,4′-diphenylmethanediisocyanate, 2,2′-diphenylmethane diisocyanate, xylylene diisocyanate,hydrogenated xylylene diisocyanate, and isophorone diisocyanate.

Examples of a compound having, in one molecule, an epoxy group (epoxycompound) which can be used as the stabilizer (B) include phenylglycidyl ether, diethyl eneglycol diglycidyl ether, bisphenolA-diglycidyl ether, hydrogenated bisphenol A-diglycidyl ether, a phenolnovolac-type epoxy resin, a cresol novolac-type epoxy resin, andepoxidized polybutadiene.

The weight average molecular weight of the stabilizer (B) is from 200 to60,000 as described above, more preferably from 200 to 30,000, and stillmore preferably from 300 to 18,000.

When the weight average molecular weight of the stabilizer (B) is withinthe above range, the stabilizer (B) moves more easily, and an effect ofimproving the moisture and heat resistance is exhibited moreeffectively.

The weight average molecular weight of the stabilizer (B) isparticularly preferably from 200 to 900. The weight average molecularweight of from 200 to 900 nearly corresponds to the number averagemolecular weight of from 200 to 900. When the weight average molecularweight is from 200 to 900, the molecular weight distribution is 1.0 insome cases. In such cases, “weight average molecular weight of from 200to 900” can be simply paraphrased as “molecular weight of from 200 to900”.

When a polymeric piezoelectric film contains the stabilizer (B), thepolymeric piezoelectric film may contain only one type of stabilizer(B), or may contain two or more types of stabilizers.

When a polymeric piezoelectric film contains the helical chiral polymer(A) and the stabilizer (B), the content (total content when the materialcontains two or more types) of the stabilizer (B) is, with respect to100 parts by mass of helical chiral polymer (A), preferably from 0.01part by mass to 10 parts by mass, more preferably from 0.01 part by massto 5 parts by mass, further preferably from 0.1 part by mass to 3 partsby mass, and particularly preferably from 0.5 parts by mass to 2 partsby mass.

When the above content is 0.01 part by mass or more, the moisture andheat resistance is further improved.

When the above content is 10 parts by mass or less, deterioration of thetransparency is further suppressed.

Examples of a preferable mode of the stabilizer (B) include a mode inwhich a stabilizer (S1) which has at least one functional group selectedfrom the group consisting of a carbodiimide group, an epoxy group, andan isocyanate group, and has a number average molecular weight of from200 to 900, and a stabilizer (S2) which has, in one molecule, two ormore functional groups of one or more kinds selected from the groupconsisting of a carbodiimide group, an epoxy group, and an isocyanategroup, and has a weight average molecular weight of from 1,000 to 60,000are used in combination. The weight average molecular weight of thestabilizer (S1) having a number average molecular weight of from 200 to900 is about from 200 to 900. The number average molecular weight andthe weight average molecular weight of the stabilizer (S1) have almostthe same values.

When the stabilizer (S1) and the stabilizer (S2) are used in combinationas the stabilizer (B), the stabilizer preferably includes a large amountof the stabilizer (S1) from a viewpoint of improving transparency.

Specifically, with respect to 100 parts by mass of the stabilizer (S1),amount of the stabilizer (S2) is preferably in a range of from 10 partsby mass to 150 parts by mass from a viewpoint of coexistence oftransparency and moisture and heat resistance, more preferably in arange of from 30 parts by mass to 100 parts by mass, and particularlypreferably in a range of from 50 parts by mass to 100 parts by mass.

Specific examples (stabilizers SS-1 to SS-3) of the stabilizer will bedescribed below.

Regarding the above stabilizers SS-1 to SS-3, the name of the compound,a commercially available product, and the like are described below.

-   -   Stabilizer SS-1∩∩∩The name of the compound is        bis-2,6-diisopropylphenylcarbodiimide. The weight average        molecular weight (in this example, simply equivalent to        “molecular weight”) is 363. Examples of the commercially        available product include “Stabaxol I” manufactured by Rhein        Chemie GmbH and “B2756” manufactured by Tokyo Chemical industry        Co., Ltd.    -   Stabilizer SS-2∩∩∩The name of the compound is        poly(4,4′-dicyclohexylmethanecarbodiimide).        Examples of the commercially available product include        “carbodilite LA-1” manufactured by Nisshinbo Chemical Inc. as        one having a weight average molecular weight of about 2,000.    -   Stabilizer SS-3∩∩∩ The name of the compound is        poly(1,3,5-triisopropylphenylene-2,4-carbodiimide). Examples of        the commercially available product include “Stabaxol P”        manufactured by Rhein Chemie GmbH as one having a weight average        molecular weight of about 3,000. Examples of those having a        weight average molecular weight of 20,000 include “Stabaxol        P400” manufactured by Rhein Chemie GmbH.

(Antioxidant)

A polymeric piezoelectric film according to the present embodiment maycontain an antioxidant. The antioxidant is preferably at least oneselected from the group consisting of a hindered phenol-based compound,a hindered amine-based compound, a phosphite-based compound, and athioether-based compound.

For the antioxidant, a hindered phenol-based compound or a hinderedamine-based compound is more preferably used. By this, a polymericpiezoelectric film having excellent moisture and heat resistance andtransparency can be provided.

(Other Components)

A polymeric piezoelectric film of the present embodiment may contain, tothe extent that the advantage of the invention be not compromised, knownresins, as represented by polyvinylidene fluoride, a polyethylene resinand a polystyrene resin, inorganic fillers, such as silica,hydroxyapatite, and montmorillonite, publicly known crystal nucleatingagents such as phthalocyanine, and other components.

When a polymeric piezoelectric film contains a component other than ahelical chiral polymer (A), the content of the component other than ahelical chiral polymer (A) with respect to the total mass of polymericpiezoelectric film is preferably 20% by mass or less, and morepreferably 10% by mass or less.

To the extent that the advantage of the invention is not compromised, apolymeric piezoelectric film of the present embodiment may contain ahelical chiral polymer other than the afore-described helical chiralpolymer (A) (namely, a helical chiral polymer (A) having a weightaverage molecular weight (Mw) from 50,000 to 1,000,000 and havingoptical activity).

From a viewpoint of transparency, the polymeric piezoelectric filmpreferably does not contain a component other than an optically activehelical chiral polymer (A).

[Crystallinity]

The crystallinity of a polymeric piezoelectric film is determined by aDSC method, and the crystallinity of a polymeric piezoelectric film isfrom 20% to 80%, and preferably from 30% to 70%, and more preferablyfrom 35% to 60%. When the crystallinity is in the above range, afavorable balance between the piezoelectricity, the transparency, andthe longitudinal tear strength of a polymeric piezoelectric film isattained, and whitening or a break is less likely to occur duringstretching, and therefore, the polymeric piezoelectric film is easilymanufactured.

When the crystallinity is 20% or more, the piezoelectricity of apolymeric piezoelectric film is maintained high.

When the crystallinity is 80% or less, deterioration of the longitudinaltear strength and transparency can be suppressed.

For example, by adjusting conditions of crystallization and stretchingduring production of a polymeric piezoelectric film, the crystallinityof the polymeric piezoelectric film can be adjusted to from 20% to 80%.

[Standardized Molecular Orientation MORc]

The standardized molecular orientation MORc of a polymeric piezoelectricfilm is from 3.5 to 15.0. The standardized molecular orientation MORc isa value determined based on a “degree of molecular orientation MOR”which is an index indicating the degree of orientation of a helicalchiral polymer (A). When the standardized molecular orientation MORc is3.5 or more, there are many molecular chains (for example, polylacticacidmolecular chains) of helical chiral polymers (A) aligned in thestretching direction, and as a result, the rate of oriented crystalsgenerated increases, whereby the polymeric piezoelectric film exhibits ahigher piezoelectricity. When the standardized molecular orientationMORc is 15.0 or less, the longitudinal tear strength of the polymericpiezoelectric film further increases.

Here, the degree of molecular orientation MOR (Molecular OrientationRatio) is measured by the following microwave measurement method.Namely, the polymeric piezoelectric film is placed in a microwaveresonant waveguide of a well known microwave transmission-type molecularorientation meter (also referred to as a “microwave molecularorientation ratio measuring apparatus”) such that the polymericpiezoelectric film plane (film plane) is arranged perpendicular to thetravelling direction of the microwaves. Then, the polymericpiezoelectric film is continuously irradiated with microwaves whoseoscillating direction is biased unidirectionally, while maintaining suchconditions, the sample is rotated in a plane perpendicular to thetravelling direction of the microwaves from 0 to 360′, and the intensityof the microwaves passed through the sample is measured to determine themolecular orientation ratio MOR.

The standardized molecular orientation MORc means a degree of molecularorientation MOR obtained based on the reference thickness tc of 50 μm,and can be determined by the following formula

MORc=(Tc/t)×(MOR−1)+1

(tc: reference thickness to which the thickness should be corrected; t:thickness of polymeric piezoelectric film)

The standardized molecular orientation MORc can be measured by a knownmolecular orientation meter, for example, a microwave molecularorientation meter MOA-2012A or MOA-6000 manufactured by Oji ScientificInstruments, at a resonance frequency around 4 GHz or 12 GHz.

The polymeric piezoelectric film has a standardized molecularorientation MORc of from 3.5 to 15.0, preferably from 4.0 to 15.0, morepreferably from 4.0 to 10.0, and further preferably from 4.0 to 8.0.

From a viewpoint of further improving adherence between a polymericpiezoelectric film and an intermediate layer, the standardized molecularorientation MORc is preferably 7.0 or less.

When the polymeric piezoelectric film is, for example, a stretched film,the standardized molecular orientation MORc can be controlled by heattreatment conditions (heating temperature and heating time) beforestretching, stretching conditions (stretching temperature and stretchingspeed), or the like.

The standardized molecular orientation MORc can be converted tobirefringence Δn which is obtained by dividing a phase difference amount(retardation) by a film thickness. Specifically, the retardation can bemeasured by a RETS100 manufactured by Otsuka Electronics Co., Ltd. MORcand Δn are approximately in a linearly proportional relationship. WhenΔn is 0, MORc is 1.

For example, when the helical chiral polymer (A) is a polylacticacid-type polymer and when the birefringence Δn of the polymericpiezoelectric film is measured at measurement wavelength of 550 nm, astandardized molecular orientation MORc of 2.0 can be converted to thebirefringence Δn of 0.005, and a standardized molecular orientation MORcof 4.0 can be converted to the birefringence Δn of 0.01.

[Product of Standardized Molecular Orientation MORc and Crystallinity]

In the present embodiment, a product of the crystallinity and thestandardized molecular orientation MORc of a polymeric piezoelectricfilm is preferably from 75 to 700. When the product is adjusted withinthe above range, the balance between the piezoelectricity and thetransparency of a polymeric piezoelectric film is favorable, and thedimensional stability is high, and deterioration of longitudinal tearstrength (that is, tear strength in a certain direction) is suppressed.

The product of the standardized molecular orientation MORc and thecrystallinity of a polymeric piezoelectric film is more preferably from75 to 600, further preferably from 100 to 500, particularly preferablyfrom 125 to 400, and particularly preferably from 150 to 300,

The product can be adjusted within the above range, for example, byadjusting the conditions of crystallization and stretching when thepolymeric piezoelectric film is manufactured.

The standardized molecular orientation MORc can be controlled byconditions (for example, heating temperature and heating time) ofcrystallization and conditions (for example, stretching temperature andstretching speed) of stretching when a polymeric piezoelectric film ismanufactured.

[Piezoelectric Constant d₁₄ (Stress-Electric Charge Method)]

The piezoelectricity of a polymeric piezoelectric film can be evaluatedby, for example, measuring the piezoelectric constant d₁₄ of thepolymeric piezoelectric film.

In the following, one example of a method of measuring the piezoelectricconstant d₁₄ by a stress-electric charge method will be described.

First, a polymeric piezoelectric film is cut to a length of 150 mm inthe direction of 45° with respect to the stretching direction (MDdirection), and to 50 mm in the direction perpendicular to the above 45°direction, to prepare a rectangular specimen. Subsequently, the preparedspecimen is set on a stage of Showa Shinku SIP-600, and aluminum(hereinafter, referred to as “A1”) is deposited on one surface of thespecimen such that the deposition thickness of A1 becomes about 50 nm.Subsequently, A1 is deposited on the other surface of the specimensimilarly, Both surfaces of the specimen are covered with A1 to formconductive layers of A1.

The specimen of 150 mm×50 mm having the A1 conductive layers formed onboth surfaces is cut to a length of 120 mm in the direction of 45° withrespect to the stretching direction (MD direction) of the polymericpiezoelectric film, and to a length of 10 mm in the directionperpendicular to the above 45° direction, to cut out a rectangular filmof 120 mm 10 mm. This film is used as a sample for measuring apiezoelectric constant.

The sample thus obtained is set in a tensile testing machine (TENSILONRTG-1250 manufactured by A&D Company, Limited) having a distance betweenchucks, of 70 mm so as not to be slack. A force is applied periodicallyat a crosshead speed of 5 min/min such that the applied forcereciprocates between 4 N and 9 N. In order to measure a charge amountgenerated in the sample according to the applied force at this time, acapacitor having an electrostatic capacity Qm (F) is connected inparallel to the sample, and a voltage V between the terminals of thiscapacitor Cm (95 nF) is measured through a buffer amplifier. The abovemeasurement is performed under a temperature condition of 25° C. Agenerated charge amount Q (C) is calculated as a product of thecapacitor capacity Cm and a voltage Vm between the terminals. Thepiezoelectric constant d₁₄ is calculated by the following formula.

d ₁₄=(2×t)/L×Cm·ΔVm/ΔF

-   -   t: sample thickness (m)    -   distance between chucks (m)    -   Cm: capacity (F) of capacitor connected in parallel    -   ΔVm/ΔF: ratio of change amount of voltage between terminals of        capacitor with respect to change amount of force

A higher piezoelectric constant d₁₄ results in a larger displacement ofthe polymeric piezoelectric film with respect to a voltage applied tothe polymeric piezoelectric film, and reversely a higher voltagegenerated responding to a force applied to the polymeric piezoelectricfilm, and therefore is advantageous as a polymeric piezoelectric film.

Specifically, in the polymeric piezoelectric film according to theinvention, the piezoelectric constant d₁₄ measured at 25° C. by astress-charge method is preferably 1 pC/N or more, more preferably 3pC/N or more, further preferably 5 pC/N or more, and particularlypreferably 6 pC/N or more. The upper limit of the piezoelectric constantd₁₄ is not particularly limited, and is preferably 50 pC/N or less, andmore preferably 30 pC/N or less, for a polymeric piezoelectric filmusing a helical chiral polymer from a viewpoint of a balance withtransparency, or the like.

Similarly, from a viewpoint of the balance with transparency, thepiezoelectric constant d₁₄ measured by a resonance method is preferably15 pC/N or less.

[Transparency (Internal Haze)]

Transparency of a polymeric piezoelectric film can be evaluated, forexample, by visual observation or measurement of haze. Internal haze forvisible light (hereinafter, also simply referred to as “internal haze”)of the polymeric piezoelectric film of the present embodiment ispreferably 50% or less, more preferably 20% or less, still morepreferably 13% or less, still more preferably 5% or less, particularlypreferably 2.0% or less, and most preferably 1.0% or less.

The lower internal haze of the polymeric piezoelectric film is, thebetter the polymeric piezoelectric film is. From a viewpoint of thebalance with the piezoelectric constant, or the like, internal haze ispreferably from 0.01% to 15%, more preferably from 0.01% to 10%, furtherpreferably from 0.1% to 5%, and particularly preferably from 0.1% to1.0%.

In the present embodiment, the “internal haze” refers to a haze fromwhich a haze caused by the shape of an external surface of the polymericpiezoelectric film is excluded.

The “internal haze” herein refers to a value measured with respect to apolymeric piezoelectric film at 25° C. in accordance with JIS-K7105.

More specifically, internal haze (hereinafter, also referred to as“internal haze H1”) refers to a value measured as follows.

That is, first, for a cell having an optical path length of 10 mm filledwith a silicone oil, a haze (hereinafter, also referred to as “haze H2”)in the optical path length direction was measured. Next, a polymericpiezoelectric film of the present embodiment is immersed in the siliconeoil of the cell such that the optical path length direction of the cellis in parallel with the normal direction of the film, and a haze(hereinafter, also referred to as “haze H3”) in the optical path lengthdirection of a cell in which the polymeric piezoelectric film isimmersed. The haze H2 and the haze H3 are both measured at 25° C. inaccordance with JIS-K7105.

Internal haze H1 is determined in accordance with the following formulabased on the measured haze H2 and haze H3.

Internal haze (H1)=Haze (H3)−Haze (H2)

Measurement of the haze H2 and the haze H3 can be performed by using,for example, a haze measuring machine [TC-HIII DPK manufactured by TokyoDenshoku Co., Ltd.,].

For the silicone oil, for example, “Shin-Etsu Silicone (trade mark),model number: KF-96-100CS” manufactured by Shin-Etsu Chemical Co., Ltd.can be used.

[Tear Strength]

The tear strength (longitudinal tear strength) of a polymericpiezoelectric film of the present embodiment is evaluated based on thetear strength measured according to the “Right angled tear method”stipulated in JIS K 7128-3 “Plastics—Tear strength of films and sheets”.

Here, the crosshead speed of a tensile testing machine is set at 200mm/min and tear strength is calculated according to the followingformula:

T=F/d

wherein T stands for the tear strength (N/mm), for the maximum tearload, and d for the thickness (mm) of a specimen.

The thickness of a polymeric piezoelectric film of the presentembodiment is not particularly restricted, and is preferably from 10 μmto 400 μm, more preferably from 20 μm to 200 μm, further preferably from20 μm to 100 μm, and particularly preferably from 20 μm to 80 μm.

<Manufacturing Method of Polymeric Piezoelectric Film>

A manufacturing method of a polymeric piezoelectric film of the presentembodiment is not particularly restricted as long as the method is amethod in which a crystallinity can be adjusted to from 20% to 80%; astandardized molecular orientation MORc can be adjusted to from 3.5 to15.0; and when a phase difference streak is evaluated by the followingevaluation method A, per a length of 1,000 mm in the direction Y, thenumber of phase difference streaks with an evaluation value of 60 ormore can be adjusted to 0, and the sum of the evaluation values of phasedifference streaks with an evaluation value of 20 or more can beadjusted to 1000 or less.

A polymeric piezoelectric film of the present embodiment can be suitablymanufactured by using, as this method, a method including a step offorming a raw material of a polymeric piezoelectric film into a film anda step of stretching the formed film. Examples of the manufacturingmethod include one described in paragraphs 0065 to 0099 of WO2013/054918,

[Molding Step]

In a molding step, a composition containing a helical chiral polymer (A)and, as needed, other components such as a stabilizer (B) is heated to atemperature not lower than the melting point Tm (° C.) of the helicalchiral polymer (A) and molded in a film shape. By this molding step, afilm containing the helical chiral polymer (A) and, as needed, othercomponents such as the stabilizer (B) is obtained.

Herein, the melting point Tm (° C.) of a helical chiral polymer (A) andthe glass transition temperature (Tg) of a helical chiral polymer (A)are values respectively obtained from a melting endothermic curveobtained by raising the temperature of the helical chiral polymer (A)under the condition of a heating rate of 10° C./min using a differentialscanning calorimeter (DSC). The melting point (Tin) is a value obtainedas a peak value of an endothermic reaction. The glass transitiontemperature (Tg) is a value obtained as an inflection point of themelting endothermic curve.

The above composition can be manufactured by mixing a helical chiralpolymer (A) and, as needed, other components such as a stabilizer (B).

Here, the helical chiral polymer (A), the stabilizer (B), and the othercomponents may be individually used singly, or two or more kinds thereofmay be used.

The above mixing may be melt kneading.

Specifically, the composition may be manufactured by charging a helicalchiral polymer (A) and, as needed, other components such as a stabilizer(B) into a melt-kneader [for example, Labo Plastomill manufactured byToyo Seiki Seisaku-sho, Ltd.] and heating the mixture to a temperaturenot lower than the melting point of the helical chiral polymer (A), andmelt kneading the mixture. In this case, a composition which has beenmanufactured by heating to a temperature not lower than the meltingpoint of the helical chiral polymer (A) and melt kneading is formed intoa film shape while maintaining the composition at a temperature notlower than the melting point of the helical chiral polymer (A).

Examples of conditions for melt kneading include conditions of a mixerrotation speed of 30 rpm to 70 rpm, a temperature of 180° C. to 250° C.,and a kneading time of 5 minutes to 20 minutes.

In the molding step, as a method for forming a composition into a film,a molding method by melt extrusion molding, press molding, injectionmolding, calendar molding, casting method is used. A composition may beformed into a film shape by a T-die extrusion molding method or thelike.

When a composition is formed into a film shape by a T-die extrusionmolding method, by adjusting the extrusion temperature and the lip tipedge radius of the T-die, an evaluation value of a phase differencestreak and the sum of evaluation values of phase difference streaks of apolymeric piezoelectric film of the present embodiment can be adjusted.For example, the extrusion temperature is preferably adjusted to from200° C. to 230° C., more preferably to from 210° C. to 225° C., and thelip tip edge radius of the T-die is preferably adjusted to from 0.001 mmto 0.100 mm, and more preferably from 0.001 mm to 0.050 mm.

In the molding step, a composition may be heated to the abovetemperature and formed into a film, and the obtained film may bequenched. By quenching, the crystallinity of the film obtained in thisstep can be adjusted.

Here, the term “quenching” refers to cooling to at least not higher thanthe glass transition temperature Tg of the helical chiral polymer (A)immediately after extrusion.

In the present embodiment, it is preferable that other processes are notincluded between molding into a film and quenching.

Examples of a method of quenching include: a method of immersing a filmin a coolant such as water, ice water, ethanol, ethanol or ethanolcontaining dry ice, liquid nitrogen or the like; and a method ofspraying a liquid spray having a low vapor pressure onto a film andcooling the film by latent heat of vaporization.

In order to continuously cool the film, it is also possible to rapidlycool the film by contacting the film with a metal roll controlled to atemperature not higher than the glass transition temperature Tg of thehelical chiral polymer (A).

The number of times of cooling may be only once or two or more times.

The film obtained in a molding step (in other words, a film to besubjected to a stretching step described below) may be a film in anamorphous state or a preliminarily crystallized film (hereinafter, alsoreferred to as a “pre-crystallized film”)

Here, the amorphous state film means a film having a crystallinity ofless than 3%.

The pre-crystallized film means a film having a crystallinity of 3% ormore (preferably from 3% to 70%).

Here, the crystallinity refers to a value measured by a method similarto the crystallinity of a polymeric piezoelectric film.

The thickness of a film (amorphous state film or pre-crystallized film)obtained in the molding step is mainly determined by the thickness ofthe polymeric piezoelectric film finally obtained and the stretchingratio, but is preferably from 50 μm to 1,000 μm, and more preferablyabout from 100 μm to 800 μm.

The pre-crystallized film can be obtained by heat-treating an amorphousfilm containing a helical chiral polymer (A) and, as needed, othercomponents such as a stabilizer (B), and crystallization.

The heating temperature T for preliminarily crystallizing an amorphousfilm is not particularly limited, and, from a viewpoint of enhancing thepiezoelectricity and transparency of a polymeric piezoelectric film tobe manufactured, it is preferable that the relationship between theglass transition temperature Tg of a helical chiral polymer (A) and thefollowing formula is satisfied and the crystallinity is set to from 3%to 70%.

Tg−40° C.≦T≦Tg+40° C.

(Tg represents the glass transition temperature of the helical chiralpolymer (A))

The heating time for preliminarily crystallizing an amorphous state filmcan be appropriately set in consideration of the standardized molecularorientation MORc and the crystallinity of an ultimately obtainedpolymeric piezoelectric film.

The heating time is preferably from 5 seconds to 60 minutes, and morepreferably from 1 minute to 30 minutes from a viewpoint of stabilizingthe manufacturing conditions. As the heating time becomes longer, thestandardized molecular orientation MORc becomes higher and thecrystallinity tends to become higher.

For example, in the case of preliminarily crystallizing a film in anamorphous state containing a polylactic acid polymer as the helicalchiral polymer (A), it is preferable to perform heating at from 20° C.to 170° C. for 5 seconds to 60 minutes (preferably from 1 minute to 30minutes).

In order to preliminarily crystallize an amorphous film, for example, acast roll adjusted to the above temperature range can be used. By usingthe electrostatic adhesion method described above, the polymericpiezoelectric film is brought into close contact with a cast roll forpreliminary crystallization, whereby it is possible to preliminarilycrystallize and adjust the peak of the thickness. For example, in thecase of adopting wire pinning to bring the entire surface of the filminto close contact, the peak of the thickness can be adjusted byadjusting the position of the electrode, material, applied voltage, andthe like.

[Stretching Process]

The stretching step is a step of stretching a film (for example, apre-crystallized film) obtained in the molding step mainly in theuniaxial direction. By this step, a polymeric piezoelectric film havinga large principal plane area can be obtained as a stretched film.

“The principal plane area is large” means that the area of the principalplane of a polymeric piezoelectric film is 5 mm² or more. The area ofthe principal plane is preferably 10 mm² or more.

It is presumed that, by stretching the film mainly in the uniaxialdirection, molecular chains of a helical chiral polymer (A) contained inthe film can be orientated in one direction and aligned at high density,thereby obtaining higher piezoelectricity, As a method of stretchinguniaxially in a continuous process, either longitudinal stretching inwhich the flow direction (MD direction) of the process and thestretching direction coincide or transverse stretching in which thedirection (TD direction) perpendicular to the flow direction of theprocess and the stretching direction coincide may be used.

In the case of stretching a film only by a tensile force such as bystretching in the uniaxial direction, the stretching temperature of thefilm is preferably in the range of about from 10° C. to 20° C. higherthan the glass transition temperature of the film (or a helical chiralpolymer (A) in the film).

The stretching ratio in the stretching treatment is preferably from twoto ten times, more preferably from three to five times, and furtherpreferably from three to four times. By this, a polymeric piezoelectricfilm having higher piezoelectricity and transparency can be obtained.

In a stretching step, when stretching (main stretching) for enhancingpiezoelectricity is performed, a film (for example, a pre-crystallizedfilm) obtained in the molding step may be stretched (also referred to assecondary stretching) simultaneously or successively in a directioncrossing (preferably perpendicular to) the direction of the mainstretching.

Herein, “successive stretching” means a stretching method, by which asheet is first stretched in a uniaxial direction, and then stretched ina direction crossing the first stretching direction.

When secondary stretching is performed in a stretching step, thestretching magnification of secondary stretching is preferably from 1time to 3 times, more preferably from 1.1 times to 2.5 times, andfurther preferably from 1.2 times to 2.0 times. By this, phasedifference streaks generated in a polymeric piezoelectric film can befurther reduced, and the tear strength can be further increased.

When a pre-crystallized film is stretched in a stretching step, the filmmay be preheated immediately before stretching so that the film can beeasily stretched. Since the preheating is performed generally for thepurpose of softening the film before stretching in order to facilitatethe stretching, the same is normally performed avoiding conditions thatpromote crystallization of a film before stretching and make the filmstiff. Meanwhile, as described above, in the present embodiment,pre-crystallization may be performed before stretching, and thereforethe preheating may be performed combined with the pre-crystallization.Specifically, by conducting the preheating at a higher temperature thana temperature normally used, or for longer time conforming to theheating temperature or the heat treatment time at the pre-crystallizedstep, preheating and pre-crystallization can be combined.

[Annealing Step]

The manufacturing method of this embodiment may include an annealing asneeded.

The annealing step is a step of annealing (heat treatment) a film(hereinafter also referred to as “stretched film”) stretched in astretching step. By the annealing step, crystallization of the stretchedfilm can be further advanced, and a polymeric piezoelectric film havinghigher piezoelectricity can be obtained.

When the stretched film is crystallized mainly by annealing, thepreliminary crystallization operation in the above molding step may beomitted. In this case, an amorphous film can be selected as a film (thatis, a film to be subjected to a stretching step) obtained in the moldingstep.

In the present embodiment, the annealing temperature is preferably from80° C. to 160° C., and more preferably from 100° C. to 155° C.

A method of annealing (heat treatment) is not particularly limited, andexamples thereof include: a method in which a stretched film is directlyheated by being in contact with a heating roll, or using a hot airheater or an infrared heater; and a method in which a stretched film isheated with a heated liquid (silicone oil or the like).

Annealing is preferably performed while applying a fixed tensile stress(for example, from 0.01 MPa to 100 MPa) to the stretched film in such amanner that the stretched film does not sag.

The annealing time is preferably from one second to five minutes, morepreferably from 5 seconds to three minutes, and still more preferablyfrom 10 seconds to two minutes. When the annealing time is five minutesor less, excellent productivity is obtained. On the other hand, when theannealing time is one second or more, the crystallinity of the film canbe further improved.

An annealed stretched film (that is; a polymeric piezoelectric film) ispreferably quenched after annealing. “Quenching” which may be carriedout in the annealing step is similar to “rapid cooling” which may becarried out in the above molding step.

The number of times of cooling may be once, or two or more times, and itis also possible to alternately repeat annealing and cooling.

<Use of Polymeric Piezoelectric Film >

A polymeric piezoelectric film can be used in a variety of fieldsincluding a loudspeaker, a headphone, a touch panel, a remotecontroller, a microphone, a hydrophone, an ultrasonic transducer, anultrasonic applied measurement instrument, a piezoelectric vibrator, amechanical filter, a piezoelectric transformer, a delay unit, a sensor,an acceleration sensor, an impact sensor, a vibration sensor, apressure-sensitive sensor; a tactile sensor, an electric field sensor, asound pressure sensor, a display, a fan, a pump, a variable-focusmirror, a sound insulation material, a soundproof material, a keyboard,acoustic equipment, information processing equipment, measurementequipment, and a medical appliance, and from a viewpoint that a highsensor sensitivity can be maintained when the film is used for a device,a polymeric piezoelectric film is preferably utilized particularly in afield of variety of sensors.

A polymeric piezoelectric film can also be used as a touch panel formedby combining the polymeric piezoelectric film with a display device. Forthe display device, for example, a liquid crystal panel, an organic ELpanel, or the like can also be used.

A polymeric piezoelectric film can also be used as a pressure-sensitivesensor, by combining the polymeric piezoelectric film with another typetouch panel (position detecting member). Examples of the detectionmethod of the position detecting member include an anti-film method, anelectrostatic capacitance method, a surface acoustic wave method, aninfrared method, and an optical method.

In this case, a polymeric piezoelectric film is preferably used as apiezoelectric element having at least two planes provided withelectrodes. It is enough if the electrodes are provided on at least twoplanes of the polymeric piezoelectric film. There is no particularrestriction on the electrode, and examples thereof to be used includeITO, ZnO, IZO (registered trade marks), IGZO, an electroconductivepolymer, silver nanowire, and metal mesh.

A polymeric piezoelectric film and an electrode may be piled up oneanother and used as a layered piezoelectric element. For example, unitsof an electrode and a polymeric piezoelectric film are piled uprecurrently and finally a principal plane of a polymeric piezoelectricfilm not covered by an electrode is covered by an electrode.Specifically, that with two recurrent units is a layered piezoelectricelement having an electrode, a polymeric piezoelectric film, anelectrode, a polymeric piezoelectric film, and an electrode in thementioned order. With respect to a polymeric piezoelectric film to beused for a layered piezoelectric element, at least one layer ofpolymeric piezoelectric film is required to be made of a polymericpiezoelectric film of the present embodiment, and other layers may notbe made of a polymeric piezoelectric film of the present embodiment.

In the case that plural polymeric piezoelectric films are included in alayered piezoelectric element, a helical chiral polymer (A) contained ina polymeric piezoelectric film in a layer has L-form optical activity, ahelical chiral polymer (A) contained in a polymeric piezoelectric filmin another layer may be either of L-form and D-form. The location ofpolymeric piezoelectric films may be adjusted appropriately according toan end use of a piezoelectric element.

For example, when the first layer of a polymeric piezoelectric filmcontaining as a main component an L-form helical chiral polymer islaminated intercalating an electrode with the second polymericpiezoelectric film containing as a main component an L-form helicalchiral polymer (A), the uniaxial stretching direction (main stretchingdirection) of the first polymeric piezoelectric film should preferablycross, especially perpendicularly cross, the uniaxial stretchingdirection (main stretching direction) of the second polymericpiezoelectric film so that the displacement directions of the firstpolymeric piezoelectric film and the second polymeric piezoelectric filmcan be aligned, and that the piezoelectricity of a laminatedpiezoelectric element as a whole can be favorably enhanced.

On the other hand, when the first layer of a polymeric piezoelectricfilm containing as a main component an L-form helical chiral polymer (A)is laminated intercalating an electrode with the second polymericpiezoelectric film containing as a main component an D-form helicalchiral polymer (A), the uniaxial stretching direction (main stretchingdirection) of the first polymeric piezoelectric film should preferablybe arranged nearly parallel to the uniaxial stretching direction (mainstretching direction) of the second polymeric piezoelectric film so thatthe displacement directions of the first polymeric piezoelectric filmand the second polymeric piezoelectric film can be aligned, and that thepiezoelectricity of a laminated piezoelectric element as a whole can befavorably enhanced.

Especially, when a principal plane of a polymeric piezoelectric film isprovided with an electrode, it is preferable to provide a transparentelectrode. In this regard, a transparent electrode means specificallythat its internal haze is 40% or less (total luminous transmittance is60% or more).

The piezoelectric element using a polymeric piezoelectric film of theinvention may be applied to the aforementioned various piezoelectricdevices including a loudspeaker and a touch panel. A piezoelectricelement provided with a transparent electrode is favorable forapplications, such as a loudspeaker, a touch panel, and an actuator.

EXAMPLES

A polymeric piezoelectric film of the present invention will bedescribed below in more details by way of Examples, provided that thepresent embodiment is not limited to the following Examples to theextent not to depart from the spirit of the present invention.

Example 1

As a helical chiral polymer (A), polylactic acid (product name: Ingeo™biopolymer, brand name: 4032D) manufactured by NatureWorks LLC wasprepared, 1.0 part by mass of the following additive X (stabilizer (B))was added to 100 parts by mass of the polylactic acid and dry blended toprepare a raw material.

The prepared raw material was placed in an extruder hopper and extrudedfrom a 2,000 mm wide T-die (lip tip edge radius 0.030 mm) while beingheated to 230° C. and brought into contact with a 50° C. cast roll for0.5 min to form a pre-crystallized film having a thickness of 150 μm(molding step).

Stretching of the obtained pre-crystallized film was started at astretching speed of 1,650 mm/min by roll-to-roll while the film washeated by being in contact with a roll heated at 70° C., and the filmwas stretched up to 3.5-fold uniaxially in the MD direction to obtain auniaxially stretched film (stretching step).

Thereafter, the uniaxially stretched film was brought into contact witha roll heated to 130° C. by roll-to-roll for 78 seconds, and thenquenched by a roll set to 50° C. Both end portions in the film widthdirection were evenly slit and then cut off to obtain a film having awidth of 1,000 mm. The film was then wound into a roll shape to obtain apolymeric piezoelectric film (annealing step).

—Additive X (Stabilizer (B))—

In Example 1, as the additive X, a mixture of Stabaxol P400 (10 parts bymass) manufactured by Rhein Chemie GmbH, Stabaxol I (80 parts by mass)manufactured by Rhein Chemie GmbH, and CARBODILITE LA-1 (10 parts bymass) manufactured by Nisshinbo Chemical Inc. was used.

Details of the components in the above mixture are as follows.

Stabaxol P400 . . . poly(1,3,5-triisopropylphenylene-2,4-carbodiimide)(weight average molecular weight: 20,000)

Stabaxol I . . . bis-2,6-diisopropylphenyl carbodiimide (molecularweight (=weight average molecular weight): 363)

Carbodilite LA-1 . . . poly(4,4′-dicyclohexylmethane carbodiimide)(weight average molecular weight: about 2,000)

Example 2

A polymeric piezoelectric film was obtained in the same manner as inExample 1 except that the extrusion temperature 230° C. from the T-diewas changed to 220° C.

Example 3

A polymeric piezoelectric film was obtained in the same manner as inExample 1 except that a T-die having a lip tip edge radius of 0.030 mmwas changed to a T-die having a lip tip edge radius of 0.003 mm.

Comparative Example 1

A polymeric piezoelectric film was obtained in the same manner as inExample 1 except that the T-die having a lip tip edge radius of 0.030 mmwas changed to a T-die having a lip tip edge radius of 0.300 mm.

The physical property values of polylactic acid used in Examples 1 to 3and Comparative Example 1 are as illustrated in the following Table 1.

TABLE 1 Polylactic acid-type resin Optical purity Resin Chirality MwMw/Mn (% ee) LA L 200,000 1.87 97.0

—Measurement of Amounts of L-form and D-form of Polylactic Acid—

Into a 50 mL Erlenmeyer flask, 1.0 g of a weighed-out sample (polymericpiezoelectric film) was charged, to which 2.5 mL of IPA (isopropylalcohol) and 5 mL of a 5.0 mol/L sodium hydroxide solution were added.The Erlenmeyer flask containing the sample solution was then placed in awater bath at the temperature of 40° C., and stirred until polylacticacid was completely hydrolyzed for about 5 hours.

After the sample solution was cooled down to room temperature, 20 mL ofa 1.0 mol/L hydrochloric acid solution was added for neutralization, andthe Erlenmeyer flask was stoppered tightly and stirred well. The samplesolution (1.0 mL) was dispensed into a 25 mL measuring flask and dilutedto 25 mL with a mobile phase to prepare a HPLC sample solution 1. Intoan HPLC apparatus 5 μL of the HPLC sample solution 1 was injected, andD/L-form peak areas of polylactic acid were determined under thefollowing HPLC conditions. The amounts of L-form and D-form werecalculated therefrom.

—HPLC Measurement Conditions—

Column:

Optical resolution column, SUMICHIRAL OA5000 (manufactured by SumikaChemical Analysis Service, Ltd.)

Measuring Apparatus:

Liquid chromatography (manufactured by Jasca Corporation)

Column temperature:

25° C.

Mobile Phase:

1.0 mM-copper (II) sulfate buffer solution/IPA=98/2 (V/V)

-   -   Copper (II) sulfate/IPA/water=156.4 mg/20 mL/980 mL

Mobile Phase Flow Rate:

Detector:

Ultraviolet detector (UV 254 nm)

—Molecular Weight Distribution—

The molecular weight distribution (Mw/Mn) of a polylactic acid used inmanufacturing each polymeric piezoelectric film of Examples 1 to 3 andComparative Example 1 was measured using a gel permeation chromatograph(GPC) by the following GPC measuring method.

—GPC Measuring Method—

Measuring apparatus:

GPC-100 (manufactured by Waters) s

Column:

SHODEX LF-804 (manufactured by Showa Denko K.K.)

Preparation of Sample:

Each polymeric piezoelectric film of Examples 1 to 3 and ComparativeExample 1 was dissolved in a solvent (chloroform) at 40° C. to prepare asample solution with the concentration of 1 mg/mL.

Measuring Conditions:

0.1 mL of a sample solution was introduced into the column at atemperature of 40° C. and a flow rate of 1 mL/min by using chloroform asa solvent, and the concentration of the sample that was contained in thesample solution and separated by the column was measured by adifferential refractometer. With respect to the molecular weight of apolylactic acid, a universal calibration curve was prepared usingpolystyrene standard samples, and the weight average molecular weight(Mw) for each polylactic acid was calculated therefrom.

<Measurement of Physical Properties and Evaluation>

With respect to the polymeric piezoelectric films of Examples 1 to 3 andComparative Example 1 obtained as described above, the phase differencestreaks were evaluated by appearance and evaluation method A describedbelow, the number of peak A and peak B, non-contact three-dimensionalsurface roughness, tear strength, piezoelectric constant (d₁₄), modulusof elasticity (45° elastic modulus in 45° direction), elongation atbreak in 45° direction (45° elongation at break), crystallinity, MORc,and internal haze were measured. Evaluation results and measurementresults are listed in Table 2.

[Internal Haze]

“Internal haze” means herein the internal haze of a polymericpiezoelectric film according to the present invention, and measured bythe following method.

Specifically, internal haze (hereinafter also referred to as “internalhaze (H1)”) of each polymeric piezoelectric film of Examples 1 to 3 andComparative Example 1 was measured by measuring the light transmittancein the thickness direction. More precisely, the haze (1-12) was measuredby placing in advance only a silicone oil (Shin-Etsu Silicone (trademark), grade: KF-96-100CS; by Shin-Etsu Chemical Co., between 2 glassplates; then the haze (H3) was measured by placing a film (polymericpiezoelectric film), whose surfaces were wetted uniformly with thesilicone oil, between two glass plates; and finally internal haze (H1)of each polymeric piezoelectric film of Examples 1 to 3 and ComparativeExample 1 was obtained by calculating the difference between the abovetwo according to the following formula:

Internal haze (H1)=haze−haze (H2)

The haze (1-12) and haze (H3) in the above formula were determined bymeasuring the light transmittance in the thickness direction using thefollowing apparatus under the following measuring conditions.

Measuring apparatus: HAZE METER TC-HIII DPK(by Tokyo Denshoku Co., Ltd.)

Sample size: Width 30 mm×length 30 mm

Measuring conditions: According to JIS-K7105

Measuring temperature: Room temperature (25° C.)

[Piezoelectric Constant d₁₄ (Stress-Electric Charge Method)] Inaccordance with “one example of a method of measuring the piezoelectricconstant d₁₄ by a stress-electric charge method” described above, thepiezoelectric constant (particularly, piezoelectric constant d₁₄(stress-electric charge method)) of a crystallized polymer film wasmeasured.

[Standardized Molecular Orientation MORc]

Standardized molecular orientation MORc was measured for each of polyenepiezoelectric films of Examples 1 to 3 and Comparative Example 1 by amicrowave molecular orientation meter MOA-6000 by Oji ScientificInstruments. The reference thickness tc was set at 50 μm.

[Melting Point Tm and Crystallinity]

10 mg of each polymeric piezoelectric film of Examples 1 to 3 andComparative Example 1 was weighed accurately and measured by adifferential scanning calorimeter (DSC-1, manufactured by Perkin ElmerInc.) at a temperature increase rate of 10° C./min to obtain a meltingendothermic curve. From the obtained melting endothermic curve, thecrystallinity was obtained.

[Tear Strength]

With respect to each of polymeric piezoelectric films of Examples 1 to 3and Comparative Example 1, the tear strength in the MD direction(longitudinal tear strength) was measured according to the “Right angledtear method” stipulated in JIS K 7128-3 “Plastics—Tear strength of filmsand sheets”.

In these examples, when the tear strength in the MI) direction is high,it means that deterioration of the longitudinal tear strength issuppressed.

In the measurement of the tear strength, the crosshead speed of atensile testing machine was set at 200 mm/min.

The tear strength (T) was calculated according to the following formula:

T=F/d

wherein T stands for the tear strength (N/mm), F for the maximum tearload, and d for the thickness (mm) of a specimen.

[Modulus of Elasticity, Elongation at Break]

For a rectangular specimen obtained by cutting each polymericpiezoelectric film of Examples 1 to 3 and Comparative Example 1 to 180mm in a direction of 45° with respect to the stretching direction (MDdirection) and to 10 mm in a direction perpendicular to the above 45°direction, the modulus of elasticity, and elongation at break in the 45°direction were measured by using a tensile testing machine STROGRAPHVD1E manufactured by Toyo Seiki Seisaku-Sho, Ltd. in accordance withJIS-K-7127.

[Appearance]

The appearance of each polymeric piezoelectric film of Examples 1 to 3and Comparative Example 1 was evaluated by the degree of brightness oflight when light is made incident from the direction perpendicular tothe main surface of the film and the emitted light is projected on thescreen and observed. Specifically, the appearance was evaluatedaccording to the following criteria.

A: Bright and dark streaks of light can hardly be recognized

B: Several bright and dark streaky lights can be observed

C: Streaky light can be observed on the whole surface

[Phase Difference Streak]

With respect to each of the polymeric piezoelectric films (thickness: 50μm) of Examples 1 to 3 and Comparative Example 1, a wide rangebirefringence evaluation system “WPA-100” manufactured by PhotonicLattice was used to evaluate phase difference streaks by the followingevaluation method A. Evaluation method A was carried out by thefollowing procedures (a) to (d).

(a) With respect to the direction Y, in-plane phase difference data of afilm was acquired at intervals of 0.143 mm to obtain an in-plane phasedifference profile. FIG. 1 is a graph illustrating the in-plane phasedifference profile of a film obtained for the polymeric piezoelectricfilm of Comparative Example 1 (from the end portion to the position of55 mm).(b) The obtained in-plane phase difference profile was subjected to FastFourier Transform, low frequency components were removed with a cutofffrequency of 0.273/mm, and then inverse Fourier transform was performed.FIG. 2 is a graph illustrating an in-plane phase difference profile ofthe film after inverse Fourier transformation (after removal of lowfrequency components) of the polymeric piezoelectric film of ComparativeExample 1.(c) For the in-plane phase difference profile after inverse Fouriertransformation, slopes of two adjacent points were calculated andconverted into a slope profile. FIG. 3 is a graph illustrating a slopeprofile of the polymeric piezoelectric film of Comparative Example 1.(d) The height from the bottom point of a valley of the obtained slopeprofile to the apex of the mountain adjacent to the valley was taken asan evaluation value of a phase difference streak.

FIGS. 4 and 5 are graphs illustrating evaluation values of phasedifference streaks for the polymeric piezoelectric films of Example 2and Comparative Example 1. As can be seen from the graphs of FIGS. 4 and5, in Comparative Example 1, many phase difference streaks wereobserved, whereas in Example 2, phase difference streaks were greatlyreduced, and almost no phase difference streaks were observed.

[Non-Contact Three-Dimensional Surface Roughness]

Non-contact three-dimensional surface roughness Sa of each of thepolymeric piezoelectric films of Examples 1 to 3 and Comparative Example1 was measured by the following method.

First, after platinum was sputtered on the measurement surface of thepolymeric piezoelectric film using sputtering apparatus (J-1000manufactured by ULVAC, Inc.), image analysis within an area of 645μm×644 μm was performed using confocal laser microscope (LEXT OLS 4000manufactured by Olympus Corporation, objective lens×20), and as aresult, non-contact three-dimensional surface roughness Sa wascalculated in accordance with ISO 25178. Specifically, this measurementwas carried out at three points in the film measurement plane, and theaverage value was taken as noncontact three-dimensional surfaceroughness Sa.

[Measurement of Thickness Peak]

Next, in order to check the undulation (thickness unevenness) of each ofthe polymeric piezoelectric films of Examples 1 to 3 and ComparativeExample 1, the thickness peak per 1,000 mm in the Y direction wasdetermined as follows.

A thickness peak was determined by using an inline film thickness meter.

When the thickness of a polymeric piezoelectric film was measured, awaveform representing the relationship between a position of the film inthe width direction and a thickness of the film was detected by aninline film thickness meter.

A waveform between a position of a film in the width directioncorresponding to the vertex of a convex portion and a position of thefilm in the width direction corresponding to the vertex of a concaveportion decreasing from the vertex of the convex portion (or between theposition of a film in the width direction corresponding to the vertex ofa concave portion and the position of the film in the width directioncorresponding to the vertex of a convex portion increasing from thevertex of the cancave portion) was set as one peak unit.

A difference between the thickness corresponding to the vertex of theconvex portion (or the concave portion) and the thickness correspondingto the vertex of the concave portion (or the convex portion) wasmeasured to calculate the peak height.

A distance between a position of the film in the width directioncorresponding to the vertex of a convex portion (or a concave portion)and a position of the film in the width direction corresponding to thevertex of a concave portion (or a convex portion) measured to calculatethe peak-to-peak distance. A peak slope is then calculated by thefollowing formula, and a peak slope is expressed as an absolute value.

|Peak slope|=(Peak height)/(Peak distance)  [Formula]:

A peak A and a peak B were determined according to the obtained peakheight and peak slope, and the number of peaks A and peaks B in each ofthe polymeric piezoelectric films of Examples 1 to 3 and ComparativeExample 1 was determined.

The peak A represents a peak having a peak height of 1.5 μm or more andthe following peak slope (namely, a value obtained by dividing the peakheight by the peak-to-peak distance) of 0.000035 or more. The peak Brepresents a peak having a peak height of 1.5 μm or more and a peakslope of 0.00008 or more.

TABLE 2 Phase difference streak/per length of 1,000 mm in direction YEvaluation Sum of Evaluation value of Evaluation evaluation Evaluationvalue of from 20 to value of Sum of values Undulation Total value offrom 40 to less than less than evaluation which is Peak A Peak B number60 or more less than 60 40 20 value 20 or more [/1000 mm] [/1000 mm]Example 1 42 0 0 6 36 658 158 12 5 Example 2 10 0 0 0 10 140 0 10 3Example 3 5 0 0 0 5 63 0 9 2 Comparative 210 4 32 142 32 6414 5848 14 5Example 1 Roughness Three- dimensional 45° 45° surface Tear elasticelongation d₁₄ × 45° Internal roughness strength d₁₄ modulus at breakelastic Crystallinity haze Sa [μm] Appearance [N/mm] [pC/N] [GPa] [%]modulus [%] MORc [%] Example 1 0.093 B 174 6.4 3.6 8.1 23.0 37.7 4.7 0.2Example 2 0.095 A 181 6.5 3.6 7.6 23.4 36.7 4.8 0.2 Example 3 0.097 A178 6.4 3.6 7.5 23.0 35.9 4.8 0.2 Comparative 0.091 C 87 6.2 3.4 3.121.1 38.4 4.6 0.2 Example 1

As listed in Table 2, in Examples 1 to 3 in which the number of phasedifference streaks having an evaluation value of 60 or more per 1,000 mmin length in the direction Y was 0 and the sum of evaluation values ofthe phase difference streaks having an evaluation value of 20 or morewas 1,000 or less, as compared with Comparative Example 1 in which thenumber of phase difference streaks having an evaluation value of 60 ormore per 1,000 mm in length in the direction Y was 1 or more and the sumof evaluation values of the phase difference streaks having anevaluation value of 20 or more was more than 1,000, the appearance wasexcellent, and the tear strength in the MD direction was large, anddeteriorate in the longitudinal tear strength was suppressed. In otherwords, it was confirmed that by reducing phase difference streaks, thetearing property was improved.

Further, in Examples 1 to 3 and Comparative Example 1, there was hardlyany difference between the confirmation result of the undulation and themeasurement result of the roughness. Therefore, it was confirmed thatthe tearing property was improved by reducing the phase differencestreak even when there was hardly any difference between theconfirmation result of the undulation and the measurement result of theroughness.

In Examples 1 to 3, it was possible to make the values of thepiezoelectric constant d₁₄, 45° elastic modulus and 45° elongation atbreak larger than those of Comparative Example 1, and it was possible tokeep the value of the d₁₄×45° elastic modulus which is a parameter ofthe sensor sensitivity larger than that of Comparative Example 1.

It was confirmed from Examples 1 and 2 that by lowering the extrusiontemperature at the time of extruding a raw material from the T-die, itwas possible to further reduce the phase difference streak and tofurther improve the tearing property.

From Examples 1 and 3 and Comparative Example 1, it was confirmed thatit was possible to further reduce the phase difference streak by makingthe lip tip edge radius smaller.

The entire disclosure of Japanese Patent Applications No. 2015-026709filed on Feb. 13, 2015 is incorporated herein by reference.

All publications, patent applications, and technical standards describedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

1. A polymeric piezoelectric film, comprising a helical chiral polymer(A) having a weight average molecular weight of from 50,000 to 1,000,000and optical activity, wherein, in the film: a crystallinity given by aDSC method is from 20% to 80%; a standardized molecular orientation MORcis from 3.5 to 15.0 when a reference thickness measured by a microwavetransmission-type molecular orientation meter is 50 μm; and when adirection parallel to a phase difference streak is a direction X, adirection perpendicular to the direction X and parallel to a main planeof a film is a direction Y, and the phase difference streak is evaluatedby an evaluation method A, per a length of 1,000 mm in the direction Y,a number of phase difference streaks with an evaluation value of 60 ormore is 0, and a sum of evaluation values of phase difference streakswith an evaluation value of 20 or more is 1000 or less, the evaluationmethod A comprising: (a) with respect to the direction Y, acquiringin-plane phase difference data of a film at intervals of 0.143 mm toobtain an in-plane phase difference profile; (b) performing fast Fouriertransformation on the obtained in-plane phase difference profile,removing low frequency components using 0.273/mm as a cutoff frequency,and then performing inverse Fourier transformation; (c) calculatingslopes of two adjacent points with respect to the in-plane phasedifference profile after inverse Fourier transformation and convertingthe slopes into a slope profile; and (d) taking a height from a bottompoint of a valley of the obtained slope profile to an apex of a mountainadjacent to the valley as an evaluation value of a phase differencestreak.
 2. The polymeric piezoelectric film according to claim 1,wherein, when evaluated by the evaluation method A, per a length of1,000 mm in the direction Y, a number of phase difference streaks withan evaluation value of 40 or more is 0, and a sum total of evaluationvalues of phase difference streaks with an evaluation value of 20 ormore is 200 or less.
 3. The polymeric piezoelectric film according toclaim 1, wherein, when evaluated by the evaluation method A, per alength of 1,000 mm in the direction Y, a number of phase differencestreaks with an evaluation value of 20 or more is 0, and a sum total ofevaluation values of phase difference streaks with an evaluation valueof 20 or more is
 0. 4. The polymeric piezoelectric film according toclaim 1, wherein internal haze for visible light is 50% or less, and apiezoelectric constant d₁₄ measured by a stress-charge method at 25° C.is 1 pC/N or more.
 5. The polymeric piezoelectric film according toclaim 1, wherein internal haze for visible light is 13% or less.
 6. Thepolymeric piezoelectric film according to claim 1, wherein the helicalchiral polymer (A) is a polylactic acid-type polymer having a main chaincontaining a repeating unit represented by the following Formula (1):


7. The polymeric piezoelectric film according to claim 1, wherein acontent of the helical chiral polymer (A) is 80% by mass or more.
 8. Thepolymeric piezoelectric film according to claim 1, wherein a product ofthe standardized molecular orientation MORc and the crystallinity isfrom 75 to
 700. 9. The polymeric piezoelectric film according to claim1, wherein internal haze for visible light is 1.0% or less.
 10. Thepolymeric piezoelectric film according to claim 1, the film containingfrom 0.01 parts by mass to 10 parts by mass of a stabilizer having oneor more functional groups selected from the group consisting of acarbodiimide group, an epoxy group, and an isocyanate group, thestabilizer having a weight average molecular weight of 200 to 60,000 (B)based on 100 parts by mass of the helical chiral polymer (A).
 11. Amethod of manufacturing the polymeric piezoelectric film according toclaim 1, the method comprising: extruding a composition containing thehelical chiral polymer (A) from a T-die having a lip tip edge radius offrom 0.001 mm to 0.100 mm at an extrusion temperature of from 200° C. to230° C. to form the composition into a film; and stretching the formedfilm.